The present invention relates to a radio frequency (RF) powered glow discharge sputtering source for mass spectrometric analysis of non-conducting solids as well as metals, alloys, semiconductors and the like.
The application of conventional direct current (DC) powered glow discharge devices for the direct analysis of conductive solids such as metals, alloys and semiconductors is well known in the art and is discussed as background to the discussion of the RF powered glow discharge devices and methods. Generally, these DC powered glow discharge devices employ low pressure, inert atmosphere plasmas to initiate cathodic sputtering of solid samples that atomizes the sample material and produces a so-called "negative glow," also called a "glow discharge." In the negative glow, sputtered material collides with electrons and metastable discharge gas atoms to produce excited state atoms and ionic species. The electromagnetic radiation produced during the decays from excited energy states to lower energy states is responsible for the "glow" phenomenon.
In a so-called "diode" design of a DC powered glow discharge device, the conducting sample being analyzed takes the form of a cathode, which is housed in a vacuum chamber along with an anode sleeve. The walls of the vacuum chamber are electrically conducting and usually also act as an anode. The vacuum chamber is filled with an inert gas such as argon. When a sufficiently high electrical potential exists between the sample forming the cathode and the chamber walls and/or sleeve forming the anode, this causes the gas to dissociate into electrons and positively charged ions, which form part of what is sometimes called the "glow discharge" or the "negative glow." Much of this atomic dissociation occurs in a particular spatial region inside the vacuum chamber, and this region is sometimes called the "discharge excitation region" or the "negative glow region."
Since the sample is the negatively charged cathode of a DC powered glow discharge device, the negative field potential attracts the positively charged ions to the sample's surface. The so-called "sputtering" phenomenon occurs when the positively charged ions are accelerated toward the surface of the sample and eventually hit the cathode surface and dislodge atoms, ions and molecules of the cathode material. By virtue of the electrical biasing created by the electrical field inside the vacuum chamber, all of the negatively charged species (e.g., electrons, negative ions, and negatively charged molecules) will be accelerated away from the negatively biased cathode surface and all of the positively charged species in the vicinity will be accelerated to the cathode surface. The vast majority of sputtered particles are not charged and can either diffuse back to the cathode surface or into the discharge excitation region. The percentage of atoms entering the discharge excitation region is a function of the discharge pressure and cathode geometry.
In theory, the sputtering process acts as a cascade of inelastic collisions with the incoming ion imparting some portion of its kinetic energy, which approaches that of the electrical potential applied to the cathode, into the cathode material's lattice structure. According to this theory, if the sputtering ion has sufficient energy and directionality, the cascade will propagate back to the surface and result in the ejection of cathode material. The atoms ejected from the cathode material diffuse into the glow discharge region and become part of the so-called glow discharge. Sputter yields, the ratio of the average number of sputtered atoms to incident ions, are a function of the relative masses of the collision partners, the incident angle and energy of the sputtering ion, and the cathode material's binding energy.
DC powered glow discharge devices are currently employed to enable the sample to be subjected to elemental analysis by atomic absorption, atomic emission, atomic mass spectrometry, and a number of laser-based spectroscopic methods. These DC powered glow discharge devices, known commonly as "sources," have been limited by the fact that the sample must be conductive in nature so that it may act as a cathode in a conventional DC diode type design of the device. In an effort to analyze nonconducting solids without dissolution, nonconducting powder samples have been mixed with a conducting powder matrix. The resulting powder mixture is pressed into a disc sample, which, because of the conductive portion, allows for the required flow of current, but which also permits the sputtering of atoms of the nonconductive material upon impingement by a discharge ion. However, the mixing of the original sample with the conducting material introduces certain problems. For example, the dilutive effect of the conducting material results in both a loss of sensitivity and an increase in the likelihood of contamination. Moreover, many nonconducting solids are not easily transformed into powders, and the transformation of the solid into a powder precludes any depth resolved analyses.
The use of a radio frequency (RF) powered, as opposed to DC powered, glow discharge in argon to sputter and ionize a solid hollow cathode sample for analysis has been described (Analytical Chemistry, 47 (9), 1528, (1975). However, the hollow cathode geometry requires that the sample itself be machined into a cylinder. Machining the sample into a cylinder requires considerable labor and prevents depth profiling analysis.
The use of any glow discharge sampling geometry in which the sample must be inserted into the vacuum chamber, automatically restricts the size and shape of the sample to be analyzed. In such instances, metals and alloys must be machined to the proper geometry. Machining and grinding eliminate the possibility of performing depth-resolved analyses. Electrically nonconductive materials such as glasses and ceramics are often nonmachineable. Nonmachineable bulk solids must first be ground into a powder and then pressed to form a solid powder sample of compatible size and shape. Additionally, the combination of powdered nonconductive samples with a conducting material results in both a loss of sensitivity and an increase in the likelihood of contamination.
A Grimm-Type high frequency powered glow discharge device (such as disclosed in Dec. 16, 1988 French Publication No. 2 616 545), which mounts the sample outside the vacuum chamber, disposes the cathode between the sample and the anode, and thus runs the risk of contamination from sputtering of the cathode material.
In order to analyze the sample in a mass spectrometer, the ionized sputtered material from the glow discharge must be transported from within the vacuum chamber of the glow discharge cell to the mass spectrometer. In a DC powered glow discharge source, a high DC voltage is applied to the walls of the vacuum chamber of the glow discharge cell in order to accelerate the charged sputtered material into the mass spectrometer. However, when this conventional method of applying a high DC voltage to both the sample and the discharge cell walls is tried in a RF powered glow discharge device, it destroys the ability of the device to maintain an analytically useful glow discharge.
Moreover, the RF glow discharge apparatus disclosed in U.S. Pat. Nos. 5,006,706 and 5,086,226 to Marcus (commonly assigned to the owner of the present application) pertain to cells maintained at ground potential. Any attempt to raise all of the RF electronics (RF generator and impedance matching network) to the same high DC electrical potential as the vacuum chamber walls have required using an isolation transformer (such as proposed at column 8, lines 8-11 in U.S. Pat. No. 4,501,965 to Douglas). This fails to provide a satisfactory solution because it causes additional problems of high voltage hazard, inconvenience, and electronic noise. For example, maintaining the RF electronics at the typical high voltage applied to the discharge cell means that the laboratory workers are subjected to the potential hazards of a five to ten thousand volt shock. Precautions needed to avoid encountering such electrical shock result in practical inconveniences in the performance of the measurements on the sample. In addition, it becomes impossible to shield the mass spectrometer system electronics adequately from RF electronic noise. Such electronic noise poses detriments to the desired limits of detection, experimental precision, and instrument calibration. Such noise also adversely affects the operation of other electronic instrumentation in the vicinity of the RF electronics because of the propagation of radio waves through this space.